Electrical machine with superconducting inductor and gas cooling of normal-conductivity windings

ABSTRACT

The electrical machine comprises a stator 1 with a superconducting winding 5 disposed in a cryostat 4 and a rotor 7 with a cooling winding 10 disposed on the rotor body. The stator also carries a compensating winding 6. The cryostat 4 is secured to the stator 1 so as to define a gap for the passage of a gaseous coolant between the external surface of the cryostat 4 and the normal-conductivity stator winding 6. The system of passages in the stator 1 and in the rotor 7 forms a circuit wherethrough the gaseous coolant flows; the coolant may circulate in a closed cooling system, said circuit forming a part of the cooling system.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to the field of electrical engineering andis specifically concerned with electrical machines with asuperconducting inductor and a gas cooling of normal-conductivitywindings.

2. Background Art

There has been proposed an electrical machine (French Pat. No.1,404,173) comprising a rotor and a stator, whereon windings aredisposed. The stator winding is made of a superconducting alloy andarranged inside a cryostat intended to maintain temperatures at whichthe superconducting state of the winding is possible. The cryostatincludes a set of coaxial cylinders interconnected at their endportions. The winding is cooled with a low-temperature liquid coolantwhich provides for the superconducting state of the winding, such aswith liquid helium (He).

The rotor, mounted in rotation bearings and made of an epoxy resinreinforced with prestressed glass fibers, carries the rotor winding madeof a normal-conductivity conductor, the rotor winding conductors havingpassages for a direct cooling.

The rotor is encompassed on the outside by the internal surface of thecryostat housing which is secured on the rotor rotation bearings. Thecryostat with the superconducting stator winding is encompassed on theoutside by a large-thickness ferromagnetic casing; the casing may be theoutside wall of the cryostat housing and protect from the externalatmospheric pressure.

The terms "external surface of the cryostat" and "internal surface ofthe cryostat" will hereinafter denote the cryostat housing surfaceswhich face the stator and the rotor respectively.

The use of a rotor winding with a direct cooling results in anon-uniform temperature distribution over the winding length, since thecoolant is heated as it flows along the cooling passage provided insidethe winding conductors. This heating causes a premature thermal ageingof the insulation in the zone of higher temperatures of the winding andhence impairs the operational dependability of the machines.

The fastening of the cryostat housing on the rotor rotation bearingscauses the rotor vibration arising from a remaining unbalance of therotor as well as the vibration originating in working members of drivenmechanisms be transmitted to the stator's superconducting winding, whichaffects the stability of its operation and impairs the operationaldependability of the machine.

SUMMARY OF INVENTION

The invention has as its aim to provide such an electrical machine witha superconducting inductor and a gas cooling of normal-conductivitywindings, wherein the relative arrangement of cooled machine units andcooling system passages ensure equalization the temperature over thelength of the stator and rotor windings along with a break of themechanical coupling which transmits vibration from the rotor to thesuperconducting winding of the stator, whereby the operationaldependability is enhanced.

The aim set forth is attained by an electrical machine with asuperconducting inductor and a gas cooling of normal-conductivitywindings, comprising a stator with a superconducting winding disposed ina cryostat and a rotor with a cooled winding disposed on its body, therotor being mounted in rotation bearings. According to the invention,the stator carries a winding which balances the electromagnetic momentof the rotor, acting upon the superconducting winding of the stator, andwhich is made of normal-conductivity conductors. The stator is providedwith a housing to whose walls the cryostat is secured so as to define agap for the passage of a gaseous coolant between the external surface ofthe cryostat and the normal-conductivity winding of the stator.

The arrangement and fastening of the cryostat on the stator breaks themechanical linkage between the rotor and the superconducting winding andthereby eliminates a transfer of vibration from the rotor and drivenmechanisms to the superconducting winding. This enhances the stabilityof its operation, while the cryostat breaks the thermal coupling throughthe coolant between the stator and rotor windings, made of anormal-conductivity conductor, which eliminates their mutual thermaleffect.

It is expedient to construct the electrical machine so that the statorand the rotor are provided with cores separated into stacks by throughradial passages and ensuring a constancy of the gap between the externalsurface of the cryostat and the normal-conductivity winding of thestator and of the gap between the internal surface of the cryostat andthe rotor winding. Openings are made in the stator housing: in thebottom part, for the inlet of a gaseous coolant, and in the end walls,for the outlet of the heated gaseous coolant, disposed above the endparts of the stator winding sections; openings are provided in the rotorbody for the passage of the gaseous coolant into the radial passages inthe rotor; and end shields defining inlet chambers for the supply of thegaseous coolant, the chambers communicating with the inner space of therotor body through said openings, are installed at the machine's frontand rear parts.

The provision of the cores and separation of them in the axial directioninto stacks and the formation of through radial passages therebetween,the provision of openings in appropriate sheets of the stator housingand rotor body and their connection in an appropriate manner to ensurethe passage of the coolant by the above-described paths allow thespecific (rated) power of the machine to be increased, while each of thewindings (of the rotor and of the stator) has its own cooling system,which augments its efficiency because of the possibility of controllingthe optimum coolant flow rates through the cooling circuits. Also, thecoolant flow which cools the normal-conductivity stator windingundergoes an intermediate cooling before its passage through thepassages between end parts of stator winding sections. This upgrades thewinding cooling efficiency.

It is advisable that partitions to restrict the axial length of the gapbetween the cryostat internal surface and the rotor winding and toprovide for the passage of the gaseous coolant in the passages betweenthe end portions of rotor winding sections be installed at the end partsof the cryostat. Partitions to separate the flow of the gaseous coolantentering the inner space of the body from the flow of the heated gaseouscoolant passing through the passages between the end portions of rotorwinding sections are installed on the rotor.

The provision of partitions restricting the axial length of the gapbetween the cryostat internal surface and the rotor winding allows theair cooled in passing through the air gap by the rotor winding and bythe internal surface of the cryostat housing to be used for cooling theend portions of the rotor winding, which affords temperatureequalization over the winding length.

It is appropriate to provide casings at the end portions of the statorhousing for collection and withdrawal of the heated gaseous coolant sothat the casings define chambers communicating through openings in thestator housing end walls with the inner space of the stator and with thepassages between the end portions of rotor winding sections.

Such casings for collection and withdrawal of the heated coolant,provided at the stator housing end parts, make it possible to isolatethe machine from the environment and to employ a closed ventilationsystem.

It is effective that ribs supported on the cryostat external wall andstator core are installed, preferably in the axial direction, in the gapbetween the cryostat external surface and the normal-conductivity statorwinding.

The provision of such ribs in the gap between the stator winding and thecryostat external surface and their fastening to the cryostat externalwall enhances the stiffness of the wall and ensures the uniformity ofthe gap around the circumference.

BRIEF DESCRIPTION OF DRAWINGS

The exact nature of the invention will now be described by a detaileddescription of embodiments thereof with reference to the accompanyingdrawings, in which:

FIG. 1 illustrates, in a longitudinal section, an electrical machineconstructed according to the invention;

FIG. 2 illustrates a modification of the electrical machine with an opencooling system;

FIG. 3 illustrates a modification of the electrical machine, whichprovides a better cooling of the rotor winding;

FIG. 4 illustrates a modification of the electrical machine with aclosed cooling system;

FIG. 5 illustrates the distribution of gaseous coolant flows amongradial passages in the stator core;

FIG. 6 illustrates a modification of the electrical machine, whichallows enhancing the stiffness of the external enclosure of thecryostat; and

FIG. 7 illustrates the withdrawal of the heated coolant from themachine.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, the proposed electrical machine comprises a stator1 having a housing 2. A cryostat 4 housing a field winding 5 made of asuperconductor is secured to end walls 3 of the housing 2. The windingis cooled in a conventional manner with the aid of a liquid coolant,such as liquid helium. The stator 1 also carries a winding 6 whichbalances the electromagnetic moment of a rotor 7, acting upon thesuperconducting winding 5. The winding 6 is constructed in aconventional manner from normal-conductivity conductors. Inasmuch as thewinding 6 carries a current equal to the rated rotor current, aconsiderable amount of heat is released in it, and the winding shouldtherefore be considered as one of the main electrical machine units tobe cooled. According to the invention, the cryostat 4 is secured on theend walls 3 of the housing 2 in such a manner that a gap is definedbetween the external surface of the cryostat and the winding 6. In theembodiment being described, rings 8 are made on the internal surface ofthe housing 2, and the winding 6 is secured in a conventional manner onthe rings 8.

The rotor 7 including a rotor body and shaft is mounted in rotationbearings 9 and carries a winding 10 constructed from normal-conductivityconductors, the winding 10 of the rotor 7 being disposed on a body 11 ofthe rotor.

An alternative embodiment of the invention is possible, wherein thestator 1 and the rotor 7 are provided with cores 12, 13 (FIG. 2), thecores being separated into stacks by through radial passages 14, 15,whose constant width is ensured by spacers 16.

The core 12 and 13 may be made of either ferromagnetic or diamagneticmaterials. When the cores are made of a diamagnetic material, slots forreliably securing the winding in a known manner may be provided therein,but if they are of a ferromagnetic material, then the electromagneticmoment will be applied to the ferromagnetic core rather than to thewindings as was the case in the above-described modifications; thisgreatly improves the operating conditions of the windings.

The introduction of the cores 12 and 13 ensures a constancy of the gapsbetween the cryostat surfaces and said cores, because cores can be madeup with a higher accuracy than placing and fastening of the windings,and a machining can also be applied when required. The constancy of theamounts of said gaps offers an uniform distribution of coolant flows viathe through radial passages 14, 15 and via the gaps between the statorand rotor windings and the corresponding walls of the cryostat.

Openings 17 to feed the gaseous coolant into the space between thestator core 12 and the stator housing 2 are provided in the bottom partof said housing. Openings 18 for the withdrawal of the heated gaseouscoolant are provided in the end walls of the housing, above the endportions of sections of the winding 6. Openings 19 for the passage ofthe gaseous coolant into the space between the core 13 of the rotor 7and the body 11 thereof are provided in the body 11 of said rotor. Endshields 20 are installed at the front and the rear part of theelectrical machine to define chambers communicating through the openings19 with the inner space of the body 11 of the rotor 7 and intended forthe supply of the gaseous coolant. Said coolant is fed into the chambersdefined by the end shields 20 through openings 21 with the aid of a fanor another conventional device. The directions of coolant flow insidethe electrical machine are shown in FIGS. 2 and 3 by arrows.

In the above-disclosed embodiment of the invention, an open system of aseparate cooling of stator and rotor elements has been described.

An alternative modification of an open cooling system is possible. For abetter cooling of the end portions of sections of the winding 10 of therotor 7, partitions 22 (FIG. 3) extending from the internal surface ofthe cryostat 4 towards the winding 10 of the rotor 7 are secured to theend portions of the cryostat 4. In the embodiment being described, thepartitions 22 are disposed radially.

The body 11 of the rotor 7 carries partitions 23 which extend in theaxial direction in the zone of the end portions of sections of thewinding 10 of the rotor 7 and are intended to shorten the axial lengthof the end shields and to separate the admitted and the withdrawn flowof the coolant from each other.

FIG. 4 shows the preferable embodiment of the invention, wherein casings24 are installed at the end portions of the housing 2 of the stator 1 todefine chambers which communicate through the openings 18 in the endwalls 3 of the housing 2 of the stator with the inner space of thelatter and with the passages between the end portions of sections of thewinding 10 of the rotor 7. This construction allows the heated gaseouscoolant to be collected from the front and the rear part of the machine,to cool it, and to feed it again for cooling the machine through theopenings 17 and 21.

The heated coolant may be cooled and fed into the openings 16 and 20with the aid of conventional means, such as air coolers and fans.

To ensure stiffness of the external enclosure of the cryostat anduniformity of the gap between the external surface of the cryostat andthe stator winding, ribs 25 (FIG. 6) are installed in this gap,preferably in the axial direction. The ribs 25 are supported on theexternal surface of the cryostat 4 and on the core of the stator 1 andmay be secured, e.g., on the external surface of the cryostat 4.

The above-described machine functions as follows.

In the embodiment of the invention, illustrated in FIG. 1, the winding 6of the stator is cooled through a convective heat exchange between thewinding, the stator housing 2, and the surrounding atmosphere. The rotor7 is cooled as well through a convective heat exchange between thewinding 10 of the rotor 7 and the surrounding atmosphere. Rotation ofthe rotor 7 gives rise to a fan effect consisting in that the rotorwinding 10, as extending in the radial direction, may be considered asan elementary fan which intensifies its own cooling. The intensity ofits cooling depends on the rotational speed of the rotor 7.

In the embodiment of the invention, illustrated in FIG. 2, feeding thecoolant into the stator housing 2 and cooling the winding of the stator1 may be accomplished with the use of an outside fan (not shown in thedrawing). In this case the coolant (air) is through the openings 17 inthe bottom part of the stator housing 2 admitted into the space betweensaid housing and the core 12 and distributed among the through radialpassages 14 as indicated by arrows in FIGS. 2 and 5. The air passingthrough the radial passages 14 cools the slot part of the winding 6 andemerges into the air gap between the winding 6 and the external surfaceof the cryostat 4, where it separates into two oppositely directedflows. Passing through the gas, it flows over the external wall of thecryostat, whose temperature is lower than that of the air, whereby itgets partly cooled and then flows into the passages between the endportions of sections of the winding 6, and after cooling these portions,flows out through the openings 18.

The rotor 7 may be cooled either with the aid of an outside fan (notshown in the drawing) or through the self-ventilation in a mannersimilar to that described above. The choice between the two said coolingmethods depends on the extent of the fan effect and in the finalanalysis on the rotation speed of the rotor 7.

When an outside fan is used, the cooling air flow is fed into the inletchambers defined by the end shields 20, wherefrom it is through theopenings 19 in the body of the rotor 7 directed from opposite ends intothe space between the core of the rotor 7 and the body 11 of the rotor.Next, the air flows via the through radial passages 15 in the core ofthe rotor 7, cools the slot part of the rotor winding, and emerges intothe air gap between the internal surface of the cryostat 4 and the core13 of the rotor 7. In the gap, the air separates into two oppositelydirected flows, and after passing through the gap, is ejected into thesurrounding atmosphere.

In the event of the self-ventilation, the air head created by therotating elements of the rotor should at least equal the head producedby the outside fan. Such a cooling method may be expedient in developingand operating electrical machines with a high rotational speed of therotor.

In the embodiment of the invention, illustrated in FIG. 3, thepartitions 22, 23 restrict the movement of air flows in the axialdirection through the gap between the internal surface of the cryostat 4and the winding 10 of the rotor 7 and direct the flows into the passagesbetween the end portions of sections of the winding of the rotor 7. Thecoolant flow passing through said gap flows over the cryostat wall whosetemperature is lower than that of the coolant (air), gets partly cooledand enters the passages between the end portions of sections of thewinding of the rotor 7. Having emerged from the passages, the air isejected into the atmosphere.

When an electrical machine is operated under such conditions thatejecting the heated coolant into the surrounding atmosphere isimpossible or when the formation of a closed cooling system with the useof air coolers and fans is needed, the heated air emerging from theopenings 18 in the end walls of the stator housing as well as the airemerging from the passages between the end portions of sections of thewinding of the rotor 7 are collected in the casings 24, wherefrom they,are via branch pipes 26 (FIG. 7), withdrawn from the machine and fed toair coolers (not shown in the drawing).

When the ribs 25 are provided, the coolant flows passing through thegaps between the corresponding surfaces of the cryostat and the statorand rotor windings are separated into parallel flows.

The proposed electrical machine ensures the equalization of temperatureover the length of the windings, which makes it possible to do away withlocal overheating as well as eliminate the transfer of rotor vibrationto the cryostat, and thereby upgrades the stability of operation of thesuperconducting winding.

INDUSTRIAL APPLICABILITY

The present invention may be employed to the best advantage in thedevelopment and operation of powerful electrical machines.

We claim:
 1. An electrical machine with a superconducting inductor andgas cooling of normal-conductivity windings, comprising a stator with asuperconducting winding disposed in a cryostat and a rotor with a cooledwinding disposed on its body, the rotor being mounted in rotationbearings, characterized in that the stator 1 carries a winding 6 whichbalances the electromagnetic moment of the rotor 7, acting upon thesuperconducting winding 5 of the stator, and which is made ofnormal-conductivity conductors, and the stator 1 is provided with ahousing 2 to whose walls the cryostat 4 is secured so as to define a gapfor the passage of a gaseous coolant between the external surface of thecryostat 4 and the normal-conductivity winding 6 of the stator.
 2. Anelectrical machine as defined in claim 1, characterized in that thestator 1 and the rotor 7 are provided with cores 12, 13, separated intostacks by through radial passages 14, 15 and ensuring a constancy of thegap between the external surface of the cryostat 4 and thenormal-conductivity winding 6 of the stator and of the gap between theinternal surface of the cryostat 4 and the winding 10 of the rotor 7,openings 17, 18 are made in the stator housing 2, in the bottom part,for the inlet of the gaseous coolant, and in the end walls, for theoutlet of the heated gaseous coolant, disposed above the end portions ofsections of the stator winding 6, openings 19 are provided in the body11 of the rotor 7 for the passage of the gaseous coolant into the radialpassages 15 in the rotor, and end shields defining inlet chambers forthe supply of the gaseous coolant, the chambers communicating with theinner space of the rotor body 11 through said openings, are installed atthe machine front and rear parts.
 3. An electrical machine as defined inclaim 2, characterized in that partitions 22 restricting the axiallength of the gap between the internal surface of the cryostat and thewinding 10 of the rotor and providing for the passage of the gaseouscoolant in the passages between the end portions of sections of therotor winding 10 are installed at the end parts of the cryostat 4, andthat partitions 23 which separate the flow of the gaseous coolantentering the inner space of the body from the flow of the heated gaseouscoolant passing through the passages between the end portions ofsections of the rotor winding 10 are installed on the rotor
 7. 4. Anelectrical machine as defined in claim 3, characterized in that casings24 are provided at the end portions of the stator housing 2 forcollection and withdrawal of the heated gaseous coolant so that thecasings 24 define chambers communicating via the openings 18 provided inthe stator body and walls 3 with the inner space of the stator and withthe passages between the end portions of sections of the rotor winding10.
 5. An electrical machine as defined in claim 4, characterized inthat ribs 25 supported on the external wall of the cryostat 4 and on thestator core 12 are installed, preferably in the axial direction, in thegap between the external surface of the cryostat 4 and thenormal-conductivity stator winding 6.